Acoustic charge transport (ACT) phenomena in III-IV semiconductor material has only recently been demonstrated. Such devices have applications as high speed analog signal processors. Delay lines have been fabricated in gallium arsenide (GaAs) substrates comprising a surface acoustic wave (SAW) transducer that launches a surface acoustic wave along an upper layer of the GaAs substrate having a transport channel formed therein. An input electrode sources charge to be transported by the propagating potential wells. A Schottky electrode receives a signal for modulating that charge. Spaced down the transport channel are often one or more non-destructive sensing (NDS) electrodes for sensing the propagating charge. Finally, there is an ohmic output electrode for removing the charge.
Initial acoustic charge transport devices comprised a thick epilayer (TE-ACT), with vertical charge confinement has been accomplished by means of an electrostatic DC potential applied to metal field plates on the top and bottom surfaces of the GaAs substrate. The field plate potentials are adjusted to fully deplete the epilayer and produce a potential maximum near the midpoint thereof. Consequently, any charge injected into the channel is confined to the region of maximum DC potential. Those skilled in the art will note that a TE-ACT separated substrate device similar to one provided by the present invention cannot be made since a top field plate would short out the electric field associated with the SAW wave in a TE-ACT substrate, and any external electric fields as well.
Lateral charge confinement (Y direction) has been achieved in several ways. Typically, a mesa is formed to define a charge transport channel. However, for thick epilayer acoustic transport devices, the mesa must be several microns in height, a fact which presents problems in fabrication and is a major impediment to the monolithic integration of conventional MESFET electronics. The rather tall (approximately 5 microns) mesa makes subsequent lithography almost impossible. Blocking potentials extending down both sides of the delay line have also been used to define the transverse extent of the channel, as has proton bombardment to render the material surrounding the channel semi-insulating.
Recently, a heterostructure acoustic charge transport (HACT) device has been fabricated using a GaAs/AlGaAs heterostructure that is similar to that of quantum well lasers and heterostructure field effect transistors (FET). A HACT device vertically confines mobile carriers through the placement of potential steps that result from band structure discontinuities. Besides providing for inherent vertical charge confinement, the HACT devices are thin film devices whose layers have a total thickness of approximately 0.25 microns, excluding a buffer layer.
HACT delay lines are characterized by a limited length due to the relatively high level of acoustic attenuation in gallium arsenide structures when compared to the length of structures fabricated in lithium niobate (LiNbO.sub.3) or quartz (SiO.sub.2), the piezoelectric substrates traditionally used for delay line applications. It would be advantageous to have an acoustic charge transport device capable of use in delay line applications displaying an increased time-bandwidth product and further capable of simplified fabrication. The present device is drawn towards such an invention.